Nanotechnology refers broadly to a field of applied science and technology whose unifying theme is the control of matter on the atomic and molecular scale, measured in nanometers, and to the fabrication of devices within that size range. Examples of nanotechnology in modern use are the manufacture of polymers based on molecular structure and the design of computer chip layouts based on surface science. Despite the great promise of numerous nanotechnologies such as quantum dots and nanotubes, real commercial applications have mainly used the advantages of colloidal nanoparticles in bulk form, such as suntan lotion, cosmetics, protective coatings, drug delivery, and stain resistant clothing.
Materials reduced to the nanoscale can suddenly show very different properties compared to what they exhibit on a macroscale, enabling unique applications. For instance, opaque substances can become transparent (copper); inert materials can become catalysts (platinum); stable materials can turn combustible (aluminum); solids can turn into liquids at room temperature (gold); insulators can become conductors (silicon). A material such as gold, which is chemically inert at normal scales, can serve as a potent chemical catalyst at nanoscales. Much of the fascination with nanotechnology stems from these unique quantum and surface phenomena that matter exhibits at the nanoscale.
As such, metal nanoparticles have received great attention due to their unique optical properties and wide range of applicability. In this context, controlling the particle-particle interaction is a major challenge to generate programmable assembly of nanoparticles which shows potential usefulness in device fabrication and detection systems. Several methods have been developed to prepare gold nanoparticle assemblies. For example, polymer single crystals, organic bridged ligands, DNA, and solid phase approaches have been used to fabricate gold nanoparticle dimer, trimer, or tetramer assemblies. Considering all of these architectures, dimers are of special interest because of their application as substrates in surface-enhanced Raman spectroscopy (SERS). Theoretical calculations have shown that nanoparticle dimers produce strong electromagnetic field enhancements which contribute efficiently to the signal enhancement in SERS sensing. Among the four listed synthetic methods, DNA-based assembly and solid phase approaches generate dimers with the highest reported yield. However, the DNA-based methods require electrophoretic separation to remove side products to achieve a high yield. On the other hand, the solid phase approaches have been limited to very small particles (<5 nm) and dimers consisting of nanoparticles with similar sizes.
As such, research and developmental efforts continue in the field of nanotechnology in the pursuit of new nano-materials exhibiting unique properties.